This invention relates to a D.C. to D.C. voltage converter, and more particularly, to a D.C. to D.C. voltage converter having a base drive circuit that reduces the increases in commutational losses of transistor switching devices normally caused by voltage increases of applied input D.C. voltages to the D.C. to D.C. voltage converter.
D.C. to D.C. voltage converters employing a saturable transformer to provide D.C. isolation between an input D.C. voltage and an output D.C. voltage are well known. A description of one such D.C. to D.C. converter is given in "Switching and Linear Power Supply, Power Converter Design" by Abraham I. Pressman, published in 1977 by Hayden Book Company, Inc., in particular, paragraph 2.4.2. of this text, entitled "Single-Transformer, Square-Wave Power Oscillator Converter (Often Referred to as a Royer Oscillator)."
The Royer oscillator essentially comprises a saturable transformer arranged with a pair of switching transistors. The switching transistors operating in their saturation condition cause the saturable transformer to be sequentially driven into its negative and positive saturation conditions to develop a square-wave type output across a secondary winding of the transformer. The switching transistors experience commutational losses as they transfer from their blocking or non-conductive state to their switching or conductive state. This transfer occurs each time the transformer is driven into its saturation conditions.
The rate at which the transformer is driven into its saturation conditions is essentially dependent upon two parameters; (1) the instantaneous rate of change of magnetic flux dB/dt related to the transformer's number of turns and the area of iron, and (2) the voltage applied to the primary winding of the transformer. The dB/dt quantity contributes to the frequency of operation of the D.C. to D.C. converter. The level of the D.C. voltage applied to the D.C. to D.C. converter contributes to the determination of the dB/dt quantity. Similarly, the level of the applied D.C. voltage typically contributes to the determination of the base drive current of the switching transistors, which, in turn determines the amount the switching transistors are driven into their saturation condition.
It is desirable that D.C. to D.C. voltage converters have the capability of receiving a plurality of input D.C. voltages whose voltage levels have a large deviation yet produce a D.C. voltage output which remains substantially the same. For example, it is desirable that if a D.C. to D.C. voltage converter receives an input D.C. voltage of 48 volts it produces a D.C. output voltage of 24 volts and that the D.C. output voltage of 24 volts is substantially maintained when the input D.C. voltage received is increased to 125 volts.
It is known that the output D.C. voltage may be maintained at approximately the same value by providing a tap on the secondary winding of the transformer to correspondingly change the desired number of turns when the input D.C. voltage is changed.
However, increases in the D.C. voltage also increase three parameters; (1) the voltage applied to switching transistors during commutation, (2) the frequency of operation of the D.C. to D.C. converter, and (3) the base drive current applied to the switching transistors. Increase in these three parameters have a cumulative effect of increasing the commutational losses of the switching transistors which may degrade the performance or even damage the transistors.
Accordingly, it is an object of the present invention to provide a D.C. to D.C. voltage converter capable of receiving D.C. voltages, whose levels have a large variation, for developing a desired D.C. output voltage, while minimizing the increase in the commutational losses of the transistor switching devices that may otherwise occur for such large variations.
It is another object of the present invention to provide a base drive current to the transistor switching devices that compensates for major and minor variations in the applied D.C. voltage so as to reduce the increase in the commutational losses of the switching transistors that may otherwise occur for both such major and minor variations.
It is another object of the present invention to provide a base drive current to the transistor switching devices that compensates for major and minor variations in the applied D.C. voltages so as to reduce the increase in the commutational losses of the switching transistors that may otherwise occur for both such major and minor variations.
These and other objects of the invention will become apparent to those skilled in the art upon consideration of the following description of the invention.